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Sandstone reservoir with strong waterdrive and crestal gas injection
This page provides a reservoir management case study for a sandstone field under strong waterdrive in which crestal gas injection techniques have been implemented.
Background and geological information
Production from this field is from several Upper Cretaceous sandstone formations. The producing zones are in pressure communication in the gas cap and aquifer but separate in the oil column. The structure is a complexly faulted anticline with a major fault separating the west and east flanks. There is minor communication across the fault. Gross thickness is 384 ft, and net-to-gross thickness is 0.7. Reservoir dip is 6° with somewhat higher dips upstructure. A tar layer exists at the original oil/water contact in the western flank of the field. The tar is 50 ft thick on the north flank and 100 ft thick on the south flank.
Porosity averages 27%, and zone permeability averages range from 1000 to 3000 md. The oil viscosity was 4 cp in the main oil column but grades higher near the tar and oil/water contact.
The field was developed and produced competitively by several operators until unitization. The primary producing mechanism was a strong waterdrive that led to some gas-cap shrinkage despite an early gas return program. Performance analysis indicated a recovery efficiency of 65% by water displacement vs. 75% by gas displacement-gravity drainage. Formation of the unit allowed initiation of crestal gas injection. Water is being produced from down-structure wells to aid in moving the gas-oil-contact (GOC) downdip. Gas-cap pressure has been increased, water influx has been virtually eliminated, and the waterdrive recovery efficiency has been replaced with better gas-drive gravity-drainage efficiency.
A double displacement process (DDP) is under way to displace water-invaded portions of the original oil column with gas. The objective is to create a gravity-stable gas front that allows the remaining oil saturation (ROS) after waterdrive to:
- Be remobilized
- Drain to the base of each zone
- Be captured in down-structure producing wells
Down-structure oil production is from multiple zones. Wells are produced primarily from a single zone to maintain control of producing gas-oil ratio (GOR) and water-oil ratio (WOR). Recompletions to other sands are performed based on well performance, cased-hole logging results, and surveillance maps.
The recovery by waterdrive and gas injection is 63% original oil in place (OOIP). The DDP is expected to add another 6% of OOIP for a total of 69%.
Field surveillance and management
There have been several major reservoir studies of the field to determine and upgrade depletion strategies. The double displacement process (DDP) was studied extensively with regional and detailed fault-block models before starting the field project. A recent sequence stratigraphy study located remaining oil and provided an improved basis for surveillance of the DDP. These studies have been used to guide infill drilling that has resulted in significant additional capture reserves. In addition, an aggressive well recompletion program has been essential in obtaining expected production and recovery performance.
A sustained surveillance program has been in place throughout the life of the field. Data on oil, gas, and water production are collected routinely each month, and annual cased-hole logging and pressure surveys are conducted. Fluid-contact mapping and material-balance models are used to monitor and balance gas-injection rates to offset fluid production and aquifer influx. The annual fieldwide cased-hole logging program to monitor fluid contacts is a very important surveillance tool in judging the effectiveness of the displacement process. Gas coning is one of the biggest operational challenges. Daily surveillance of GORs by field personnel and active involvement of the engineering staff combine to maintain maximum oil production rates. Water has swept heavier oil from deeper depths to shallower depths, reducing well productivity and slowing the gravity drainage process. Frequent measurements of oil gravities are used to understand and predict producing characteristics.
Heavy oil creates oil/water separation problems, which leads to injection of contaminants in saltwater disposal wells. This results in the need for frequent saltwater disposal well cleanout workovers. Keeping the surface-handling facilities clean minimizes these workovers. Techniques include:
- Regularly scheduled tank inspections and cleanouts
- Appropriate internal tank piping/oxygen exclusion
- Well-managed water chemistry surveillance and chemical programs
- The use and maintenance of appropriate transfer pumps
Periodic backflowing of gas-injection wells has had a significant positive impact in maintaining high levels of injectivity and minimizing gas-injection pressures.
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